The introduction of adenovirus as a vector for in vivo delivery of transfected genes used in therapeutic treatments, as well as in vaccines, has resulted in a realm of highly focused research and development that will ultimately be applied to many different disease targets. Clinical studies used to estimate dosage levels of transfected adenovirus particles for appropriate efficacy are highly dependent upon techniques that can provide accurate and reproducible methods to assess viral particle concentration. Several methods for determining adenovirus particle concentration using spectroscopic methods with various types of sample pre-treatments have been published in the literature (see. e.g., Lawrence and Ginsberg, 1967, J. Virol. 1: 851-867; Maizel et al., 1968, Virology 36: 115-125; Liebermann and Mentel, 1994, J. Virological Methods 50, 281-292; Huyghe et al., 1995, Hunan Gene Therapy 6, 1403-1416; Mittereder et al., 1996, J. Virol. 70 (11), 7498-7509; Shabram et al., 1997, Human Gene Therapy 8, 453-465). Maizel et al. (id.) in particular established a correlation between protein content of adenovirus preparations and absorbance at 260 nm and reported an absorptivity for purified adenovirus preparations of 1.1×1012 viral particles per absorbance unit (AU). This method and absorptivity value have been cited extensively in the literature as the basis for quantitating adenovirus particle concentration. However, while the simplicity of this spectroscopic method is attractive, application of this methodology has resulted in poor accuracy when compared to direct evaluation of protein concentration, and higher than expected intra- and inter-assay variability than would be expected from a spectroscopic method. No one method, including the methodology forwarded by Maizel et al. (id.) has been applied in a consistent enough manner to provide a “universal” approach for defining adenovirus concentration. Furthermore, many of these methods produce conflicting results. In fact, Mittereder et al. (1996, J. Virol. 70 (11), 7498-7509) state in a work intended to evaluate concentration and bioactivity of adenovirus, that the accuracy of the absorptivity of adenovirus preparations at 260 nm is an area that needs to be further investigated. It would be desirable to have at hand a simple, precise and reproducible method for determining virus particle concentrations, especially viral particles which are being utilized in gene therapy and/or gene vaccination regimes, such as a representative recombinant adenovirus vector. The present invention addresses and meets these needs by disclosing a simple, precise and easily reproducible method for determining viral particle concentration using spectroscopic methods.